The present invention relates generally to a digital printer and, more particularly, to a color digital printer using imaging stations adjacent both the tensioned and non-tensioned sides of a photoreceptor belt.
In recent years, digital color printers have been increasingly utilized to produce output copies from input video data representing original image information. The printer uses a plurality of imagers, either raster output scanners (ROSs) or LED print bars, to expose the charged portions of a photoconductive member to record an electrostatic latent image thereon. Generally, a raster output scanner incorporates a laser for generating a collimated beam of monochromatic radiation. The laser beam is modulated in conformance with the image information. The modulated beam is incident on a scanning element, typically a rotating polygon having mirrored facets. The light beam is reflected from each facet and thereafter focused to a spot on the photosensitive member. The rotation of the polygon causes the spot to scan linearly across the photoconductive member in a fast scan (i.e., line scan) direction. Meanwhile, the photoconductive member is advanced relatively more slowly in a slow scan direction which is orthogonal to the fast scan direction. In this way, the beam scans the recording medium with a plurality of scan lines in a raster scanning pattern. The time between the formation of each scan line is constant due to the rotating nature of the polygon. Thus, such operation is characterized as synchronous. The light beam is intensity-modulated in accordance with an input image serial data stream, at a rate such that individual picture elements ("pixels") of the image represented by the data stream are exposed on the photoconductive member to form a latent image, which is then transferred to an appropriate image receiving medium such as paper. In an LED print bar system, video data is applied to drive circuits which control the instant on/off time of the individual diodes comprising the print bar. A modulated output is coupled through a linear lens array onto the surface of the photoreceptor medium to form the image scan lines. Each image line is formed instantly; the time between scan lines can be constant as in the case of the ROS using a polygon scanner, or can vary if desired. The latter type of operation is characterized as synchronous.
Color digital printers may operate in either a single pass or multiple pass mode. In a single pass, process color system, three imagers are positioned adjacent to a moving photoreceptor surface and are selectively energized to create successive image exposures, one for each of the three process colors, cyan, magenta and yellow. A fourth black imager is usually added. A color digital printer may also operate in a highlight color mode wherein one or two colors and black are exposed.
In a multiple pass system, each image area on the photoreceptor surface must make at least three revolutions (passes) relative to the transverse scan lines formed by the modulated beam generated by the imagers. With either system, each image is typically formed within a tight tolerance of .+-.0.05 mm. Each color image must be registered in both the photoreceptor process direction X (skew registration) and in the direction Y parallel to the process direction (referred to as fast scan or transverse registration). Registration in the transverse direction of a single pass ROS imager system is known in the prior art and registration techniques are disclosed in copending U.S. Ser. No. 07/635,835 filed on Jan. 3,1991, and U.S. Ser. No. 07/807,927 filed on Dec. 16, 1991, both assigned to the same assignee as the present invention. Registration techniques for LED print bar imagers are disclosed in U.S. Ser. No. 07/807,931 and U.S. Ser. No. 07/862,150. The contents of these applications are hereby incorporated by reference. Further references located during a prior art search are briefly summarized below.
U.S. Pat. No. 4,933,727 to Mizuma et al. discloses a color recording apparatus which forms color image precisely overlapping each other in a record medium. Record medium conveying motion and latent image forming motion are precisely synchronized with each other by a predetermined timing relation, so as to have the forward movements of the record medium coincide to the start of the electrostatic latent image forming operation to form an electrostatic latent image on a photoconductor. A photoconductor belt 211 is designed in a vertical manner as shown in FIGS. 1-2. See Col. 3, line 66-Col. 4, line 25.
U.S. Pat. No. 4,752,804 to Ohno discloses a color image forming apparatus in which toner images are successively transferred from a plurality of image bearing members to a predetermined transfer material. A control system is included in order that a peripheral speed of the imaging member is constant. The control of the constant speed is effected by a pulse generated from an encoder by a revolution of a drive motor which is counted at a predetermined time. A control signal is output from a driver to the motor through a motor control CPU to a D/A converter so that the revolution of the motor coincides with a predetermined pulse number. The apparatus provides a detecting and coinciding means which detects a phase angle during rotation of the image bearing members.
U.S. Pat. No. 4,965,597 to Ohigashi et al. discloses a color image recording apparatus which superimposes a plurality of images having different colors to form a composite image. Registration marks are formed on a recording medium at equal pitches as it is transported through an image formation device. The apparatus is comprised: (1) a sensor for sensing the registration marks; (2) an edge sensor; and (3) a timing control means for controlling an energizing means at a time a latent image is formed on a recording medium. Each pitch between the registration marks is measured at the sensor means. Steps are taken each time the registration mark is sensed to detect the discrepancy between the number of lines of the latent image formed up to the time the registration mark is sensed, and the number of lines established when the registration marks are formed in the beginning.
U.S. Pat. No. 4,252,432 to Ophey discloses a control system for an electrophotographic copier having an endless photoconductive belt. The photoconductive belt has at least one marking which excites a sensor each time the marking passes a fixed point in the belt path. The belt is provided with: (1) a first drive means for driving the photosensitive belt and for moving the belt into a first section alternately at a high constant speed when scanning and exposing, and at a low constant speed or stopping when exposing and scanning are returned to an original position; (2) a second drive means for moving the belt in a second section at a constant speed. See col. 2, lines 26-58. The movement of the belt is measured electronically by a pulse generator coupled to a belt transport roller and a counter.
U.S. Pat. No. 4,803,515 to Hoshino et al. discloses an image forming apparatus wherein a time interval is required for an image bearing member to move from a latent image forming position to a transfer position. The time interval can be an integer multiple of a period of a drive non-uniformity which is inherent in a driver.
U.S. Pat. No. 4,901,110 to Tompkins et al. discloses an electrophotographic print engine with a photosensitive belt stretched over a plurality of rollers such that a developing position on the belt is disposed in a vertical plane.
U.S. Pat. No. 4,801,978 to Lama et al. discloses a printer which uses an encoder to detect spatial displacements of indicia affixed to a photoreceptor surface to adjust on/off timing of the write bars in order to adjust for effects of photoreceptor vibration.
These prior art applications are characterized by having imager stations, either ROS or print bars, on the same side of a photoreceptor belt. The belt is driven around a main drive roller, which is driven by a synchronous speed motor. The belt system requires that a tension roller be added to maintain the belt in a flat orientation throughout its rotation. The belt thus acquires a tensioned and non-tensioned instant surface, depending upon its location relative to the drive and tension rollers. With this arrangement, the belt surface speed varies at different points, depending upon the rotation position of the drive roller and motion of the tension roller. As will be seen, the tensioned side of the belt has a speed which varies in a synchronous or periodic fashion. The speed of the belt on the non-tension or slack side, varies in a non-synchronous or non-periodic fashion. This situation is shown in FIG. 1.
FIG. 1 shows a configuration of a photoreceptor belt 10 being driven by a drive roller 12, rotated in the indicated direction by a synchronous speed motor 13. Roller 14 provides tension to the belt by a spring 15 or equivalent tensioning means and maintains it in a generally flat orientation. In the embodiment shown, the top surface 16 of belt 10 is the tensioned side, while the bottom surface 18 is the non-tensioned side. In a single pass color system, two or more imagers are required to be located adjacent the belt surface (two or more imagers are used in highlight color systems, usually four in process color). Belt 10 will have a number of exposure frames or pitches available to be written on by the imagers. The velocity of the belt, V(t), at the tensioned side of the belt, varies due to an inherent runout of the drive roller. Since the drive roller runout is periodic over the circumference, the resulting velocity variation is also periodic with the drive roller circumference. As shown by the sketch, the variation is periodic with a period equal to the circumference of the drive roller 12. The tensioned side of the belt thus has a speed which, at any given point, 1-6, etc., will vary depending upon its location at any given time. Thus, if points 2, 4 and 6, for example, are spaced apart by a distance which is an integer multiple of the period of the drive roller; e.g. a distance NC, where N is an integer (here N=1), the velocity of the belt at points 2, 4 and 6; e.g. V.sub.1 (t), V.sub.2 (t), V.sub.3 (t) at any time t will be equal. The belt is thus characterized as having phase synchronicity at these points. If imagers 19, 20 and 21, driven by signals from Electronic Sub System (ESS) 22 are arranged to form scan lines at these three points, all points on the line formed at that portion of the belt will be moving at the same velocity; e.g. V.sub.1 (t)=V.sub.2 (t)=V.sub.3 (t).
Considering next the velocity V(t') of the belt on the non-tensioned side 18 of the belt, the velocity at point 7-12 also has runout variations, due to the motion of tension roller 14. These variations are non-periodic and separation of imaging stations at any of points 7-12 by NC values will not establish the phase synchronicity possible with the imagers on the tensioned side of the belt.
It would be desirable to have the option of placing imaging stations anywhere along the periphery of a belt-type architecture. This arrangement would afford wider latitude in design of the housing and permit more efficient use of the available space. The problem, as set forth above, is the registration of images on the non-tensioned side of the belt with those on the tensioned side, while forming color images in a single pass. According to a first aspect of the present invention, a technique is provided which includes measurement of the actual speeds at synchronous operating imaging stations on the tensioned side of the belt and comparison of these speeds with the speeds at imaging stations at the non-tensioned side of the belt. Error signals are generated which represents the difference in the speeds. These signal are used to control the drive circuits to an asynchronous imager, such as an LED print bar, to control the pulsed on/off time (line write time) of the print bar and insure that the spacing between scan lines are the same value for all of the imagers. According to a second aspect of the invention, single pass color images are formed on the tensioned side of the belt which are not separated by an NC value. More particularly, the present invention relates to an imaging system for forming multiple registered latent images on a photoreceptor belt in a single pass comprising:
means for driving the belt around at least a drive roller having a circumference C and a tension roller thereby creating a tensioned and non-tensioned surface of the belt,
at least one imaging station located on each said tensioned and non-tensioned surface of the belt, each imaging station including an imager which is addressed by video data signals to form a plurality of image scan lines across the surface of the photoreceptor moving therepast at a characteristic speed, at least one of said imagers comprising a print bar capable of asynchronous operation located adjacent the non-tensioned surface,
means for comparing belt speed at each of said imaging stations and for generating a signal representing the difference between said signals, and
control means for varying the write pulse time of said print bar so that said scan lines formed by said print bar are in registration with the scan lines formed by the imagers on the tensioned side of the belt.